1. Field of the Invention
The present invention relates to a multistage evaporator plant featuring combined vapor compressor and heat pump for regeneration of the heat contained in the vapors.
2. Brief Description of the Background of the Invention Including Prior Art
At present multistage evaporator plants, evaporator plants with vapor compressors or combined multistage evaporators with vapor compressors are employed for reducing the process energy use in thermal processes for separating materials. The energy savings are achieved in multiple stage evaporators by employing the heat contained in the vapors of the individual evaporator stages for heating the next stage in each case. The vapors of the last evaporator stage are liquefied in vapor condensers and the condensate is removed into the environment, since it is in general not possible to economically use the heat contained in these vapors. Possibly the evaporation enthalpy of the vapors can be used for a preheating of the solution to be concentrated, however since the amount of evaporation enthalpy is always larger than the amount of heat which can be transferred to the solution, the largest part of the vapor heat has to be removed into the environment without use. The optimum number of stages and thus the possible energy savings with multistage evaporators is determined by the technical, thermodynamical and especially the economical constraints, since the evaporation processes can only be performed within certain temperature ranges determined by the properties of the materials, the available process steam temperatures and the material data of the solutions to be evaporated and since further the capital and maintenance expenditures with increasing number of stages begin to increase in a larger proportion compared to the energy savings.
Another possibility of economically employing high value energy in evaporation processes comprises to compress the vapors to the higher heating vapor pressure and to feed them back as process steam to the evaporator, which is in most cases the apparatus where they were generated. It is known in the art to compress the vapors by way of mechanical compressors as well as with steam jet injectors. The compression of the vapors with mechanical compressors is however economically and thermodynamically only reasonable in cases where large amounts of vapor are processed and where the ratio between the pressure of the vapors and the pressure of the heating steam is as small as possible. In producing plants the advantageous temperature difference between the vapors and the heating steam is 10 K. to 20 K. and beyond these temperature differences mechanical vapor compressors do not work economically.
Instead of the expensive and in many cases considerable maintenance requiring mechanical compressors the less expensive and sturdier steam jet injectors can be employed. The injector compressor requires a considerable amount of booster steam for each kg of vapors and the amount of booster steam required becomes the larger the smaller the difference between the vapor temperature and the booster steam temperature is and the larger the difference between the vapor and heating steam temperature is. However, since the required amount of heating steam does not considerably exceed the amount of vapors, it can be concluded that only part of the vapors can be compressed while the balance is removed to the environment as waste heat unless there is some other use.
Therefor, since with increasing temperature difference between the vapors and the heating steam the energy use for the operation of the compressors and/or the technical expenditures increase rapidly, a combination of multistage evaporators and vapor compressors is uneconomical and does not make sense in view of energy use if the aim is to use as far as possible the difference between process steam and environmental temperature in the evaporator part and further to lift up the vapor temperature as low as possible of the last stage by way of a vapor compressor to the level of the process steam. The advantages of the one system as saving of energy by using the large temperature difference between the vapors of the last stage and the heating steam for the first stage of the multistage evaporator can completely balance the advantages of the other system as the less the temperature difference between vapors and heating steam, the less is the energy expenditure in the vapor compressors.